The Standard Electrical Dictionary Part 102

Magnetic Quant.i.ty.

The magnetism possessed by a body; it is proportional to the action of similar poles upon each other, or to the field produced by the pole in question. It is also called the strength of a pole.

The force exercised by two similar poles upon each other varies with their product and inversely with the square of the distance separating them; or it may be expressed thus (m * m) / (L^2). This is a force, and the dimensions of a force are ML/(T^2). Therefore, (m^2)/(L^2) = ML/(T^2) or m = (M^.5)*(L^1.5)/T.

351 STANDARD ELECTRICAL DICTIONARY.

Magnetic Reluctance.

The reciprocal of permeance; magnetic resistance; the relative resistance to the pa.s.sage of lines of force offered by different substances. The idea is derived from treating the magnetic circuit like an electric one, and basing its action on magneto-motive force acting through a circuit possessing magnetic reluctance.

Magnetic Reluctivity.

The reciprocal of magnetic permeability, q. v.

Synonym--Magnetic Resistance.

Magnetic Retentivity.

The property of steel or hard iron by which it slowly takes up and slowly parts with a magnetic condition--traditionally (Daniell) called coercitive force.

Magnetic Rotary Polarization.

If a plane polarized beam of light is sent through a transparent medium in a magnetic field its plane of polarization is rotated, and this phenomenon is denoted as above. (Compare Refraction, Electric, and see Electro-magnetic Stress.) This has been made the basis of a method for measuring current. A field of force varies with the current; the polarization produced by such field is therefore proportional to the current. (Becquerel & Rayleigh.)

A plane polarized beam of light pa.s.sing through the transparent medium in the magnetic field by the r.e.t.a.r.dation or acceleration of one of its circular components has its plane of polarization rotated as described.

The direction of the lines of force and the nature of the medium determine the sense of the rotation; the amount depends upon the intensity of the field resolved in the direction of the ray, and on the thickness and nature of the medium.

Magnetic Saturation.

The maximum magnetic force which can be permanently imparted to a steel bar. A bar may be magnetized beyond this point, but soon sinks to it.

The magnetism produced in a bar is prevented from depolarization by the retentivity or coercive force of the bar. The higher the degree of magnetization the greater the tendency to depolarization.

It is also defined as the maximum intensity of magnetism produced in a paramagnetic substance by a magnetic field as far as affected by the permeability of the substance in question. The more lines of force pa.s.sed through such a substance the lower is its residual permeability.

It is a.s.sumed that this becomes zero after a certain point, and then the point of saturation is reached. After this point is reached the addition of any lines of force is referred entirely to the field and not at all to the permeability of the substance. But such a zero is only definable approximately.

Magnetic Screen.

A box or case of soft iron, as thick as practicable, for protecting bodies within it from the action of a magnetic field. The lines of force to a great extent keep within the metal of the box on account of its permeability, and but a comparatively few of them cross the s.p.a.ce within it.

Such screens are used to prevent watches from being magnetized, and are a part of Sir William Thomson's Marine galvanometer.

A magnetic screen may be a sphere, an infinite or very large plane, or of the shape of any equipotential surface.

Synonym--Magnetic s.h.i.+eld.

352 STANDARD ELECTRICAL DICTIONARY.

Magnetic Self-induction.

The cause of a magnet weakening is on account of this quality, which is due to the direction of the lines of force within a magnet from the positive towards the negative pole. "A magnet thus tends to repel its own magnetism and to weaken itself by self-induction." (Daniell.)

Magnetic Separator.

An apparatus for separating magnetic substances from mixtures. Such separators depend on the action of electro-magnets. In one form the material falls upon an iron drum, magnetized by coils. Any magnetic substance adheres to the drum and is thereby separated. They are used by porcelain makers for withdrawing iron particles from clay, by machinists to separate iron filings and chips from bra.s.s, and for similar purposes.

Fig. 224. MAGNETIC SEPARATOR.

Magnetic Sh.e.l.l.

A theoretical conception of a cause of a magnetic field or of a distribution of magnetism. If we imagine a quant.i.ty of very short magnets arranged in contact with their like poles all pointing in the same direction so as to make a metal sheet, we have a magnetic sh.e.l.l.

Its magnetic moment is equal to the sum of the magnetic moment of all its parts. If the sh.e.l.l is of uniform strength the magnetic moment of a unit area gives the strength of the sh.e.l.l; it is equal to the magnetic quant.i.ty per unit of area, multiplied by the thickness of the sh.e.l.l.

If its strength is uniform throughout a magnetic sh.e.l.l is called simple; if its strength varies it is termed complex.

Emtage thus defines it: A magnetic sh.e.l.l is an indefinitely thin sheet magnetized everywhere in the direction normal to itself.

Magnetic Sh.e.l.l, Strength of.

The magnetic quant.i.ty per unit of area of the sh.e.l.l multiplied by the thickness of the sh.e.l.l.

353 STANDARD ELECTRICAL DICTIONARY.

Magnetic s.h.i.+eld.

In general a magnetic screen, q. v. Sometimes a strong local field is made to act as a s.h.i.+eld, by its predominance overcoming any local or terrestrial field to which the needle to be protected may be exposed.

Magnetic Shunt.

The conception of a magnetic circuit being formed, the shunt is a corollary of the theory. It is any piece of iron which connects points of a magnet differing in polarity, so as to divert part of the lines of force from the armature or yoke. The shunt is especially applicable in the case of horseshoe magnets. Thus a bar of iron placed across from limb to limb a short distance back from the poles would act as a shunt to the armature and would divert to itself part of the lines of force which would otherwise go through the armature and would weaken the attraction of the magnet for the latter. In dynamos a bar of iron used as a magnetic shunt has been used to diminish the lines of force going through the armature and hence to weaken the field and diminish the electro-motive force. By moving the shunt nearer or further from the poles the dynamo is regulated.

In the cut the projections between the yoke and poles of the magnet shown act as a shunt to the yoke, taking some lines of force therefrom.

Fig. 225. MAGNETIC SHUNT.

Magnetic Storms.

Terrestrial magnetic disturbances sometimes covering very wide areas, and affecting the magnetic declination and inclination. One such disturbance was felt simultaneously at Toronto, Canada, the Cape of Good Hope, Prague and Van Diemen's Land. (Sabine.)

354 STANDARD ELECTRICAL DICTIONARY.

Magnetic Strain.

The strain produced by magnetic lines of force in substances exposed to their action. It is observed in substances placed between the poles of a strong electro-magnet, and evinces itself in the alteration of the optical properties of transparent substances.